In a magnetic resonance imaging (MRI) apparatus, a gradient magnetic field (magnetic field having a different magnetic field strength depending on the position) is provided for each of three axes, namely, X-, Y-, and Z-axes of the human body for the purpose of designating desired cross-sectional position to be imaged and obtaining position information within the cross-section. As efforts to improve image quality of the MRI apparatus, enhancement of the magnetic field strength of the gradient magnetic field and pulsed high-speed driving of the gradient magnetic field are performed. For these purposes, a current of several hundred amperes (A) at several hundred Hz is supplied to a coil that generates the gradient magnetic field. Such a high-speed temporal magnetic field change generates eddy currents in a conductor near the coil. The eddy currents distort the gradient magnetic field and lead to deterioration of image quality.
There is a need to perform current control for correcting magnetic field distortion due to eddy currents in order to prevent deterioration of image quality. For example, conventional correction methods include a method for disposing a correction coil (to correct the magnetic field leaking from the gradient magnetic field coil) at outside of the gradient magnetic field coil and for correction the influence due to the eddy currents by temporally changing the current flowing through the correction coil. This method, however, would increase the cost since the correction coil needs to be newly disposed, and would produce a need to change the current in the correction coil in response to a temperature change in the conductor that generates eddy currents.
Moreover, there is another method for correction using a fixed filter having an appropriate time constant designed by measuring a nuclear magnetic resonance (NMR) signal when the current in the gradient magnetic field coil is changed stepwise to zero with a phantom positioned in the MRI at installation. However, this method using the fixed filter has difficulty in perfectly correcting the magnetic field distortion due to eddy currents.